Steel alloy of great tenacity, particularly suitable for chips removing cutting tools



v EGL] STEEL ALLOY OF GREAT TENACITY, PARTICULARLY SUITABLE June 9, 1959 FOR CHIPS REMOVING CUTTING TOOLS Filed Oct 8, 1956 l l I I l i l l l vw W United States atent Ofifice 2,390,113 Patented June 9, 1959 STEEL ALLOY OF GREAT TENACITY, PARTICU- LARLY SUITABLE FOR CHIPS REMOVING CUT- TING TOOLS Rudolf Egli, Milan, Italy, assignor to Interstahl Etablissement, Vaduz, Liechtenstein Application October-s, 1956, Serial No; 614,524

Claims priority, application Italy January 17, 1956 1 Claim. (Cl. 75-126) This invention relates to malleable and pourable steel alloys, having great tenacity and at the same time great hardness, resistance to wear and a mechanical strength when hot, especially suitable for chip producing cutting tools.

Cutting tools for machining off chips, which must provide for a high output, are made at present from two types of steel alloy which differ materially from each other, and more particularly:

(1) Alloys with a chromium content not exceeding 4%, which further include additions of the so-called high grade alloy components such as tungsten, molybdenum, vanadium and the like, and

(2) Alloys with a very high chromium content, acting as a base of the alloy, with additions of relatively low quantities of high grade alloy components.

These last steel alloys for cutting tools have occupied an established position, as compared with the first class of steel alloys, only in recent times, since, the output being equal or even greater, they are more advantageous economically. The production of such alloys dates back a long time, however without aiming at the final object of obtaining a steel alloy for manufacturing chip producing cutting tools.

While for the steel alloys having a chromium content of less than 4% the additions of high grade alloy components were determined empirically, more recent research has shown, that there are certain rules which, in connection with the class of high chromium content steel alloys having relatively small additions of the high grade alloy components, govern the quantitative proportions of the alloy components for tool steel alloys.

It is therefore necessary to maintain certain optimum ratios between the contents of the high grade alloy components and the chromium base, and further to adapt the heat treatment to the desired composition.

The greatest difliculty in the development of these steel alloys was presented by the research for the proper elements to be added to the alloy, as well as by the selection of a suitable heat treatment to follow, so as to give the tools produced from that alloy sufiicient resistance to heat and the necessary resistance to wear during machining work. Such steel alloys usually have alloying elements such as tungsten, molybdenum, vanadium and cobalt, the total content of these elements being between 60% and 100% of the chromium in the alloy, and the tungsten content further being more than the total of the contents of molybdenum, vanadium and cobalt in the alloy.

A further type of these steel alloys with a high chromium content which is intended to provide an improve ment in the resistance to wear includes from 8% to 13% chromium, the tungsten content being below 24% and the alloying components such as tungsten, molybdenum and vanadium, being together in a quantity which is lower than the chromium content, the vanadium and molybdenum content together being less than the tungsten content while the quantity of dissolved carbon must beabove 1%.

It should be noted from these considerations that heretofore an increase in the resistance to heat and resistance to wear was mainly aimed at. However too little attention has been given to the most important quality in a high output chip removing cutting tool, which is tenacity.

In a more recent development of the technique of machining olf chips, it was considered essential for the tool steel alloys to possess a certain amount of hardness and resistance to wear, and in addition, very high tenacity for very high output tools for machining off chips. In practice it has been found that the steel alloys with a high chromium content, due to this requirement for hardness and resistance to wear, have a greater tenacity and give therefore a better output.

It is an object of the present invention to provide a chromium base steel alloy for the manufacture of very high output cutting tools for machining off chips which have, apart from great hardness, resistance to wear and resistance to heat, a great tenacity, such as results of hardness, resistance to wear and resistance to heat which must be at least as high as for the present day steel alloys which are similarly classified.

Experience in accordance with this invention has shown that a greater tenacity is obtained, other than from a greater chromium content as above mentioned, also by keeping the number of additive elements in the alloy as low as possible and maintaining the content of such elements within the minimum allowable limits.

More particularly it has been found that in the chromium base steel alloys which are subjected to a heat treatment to achieve the normal hardness and heat resistance required, only a small number of the above mentioned additive elements of the alloy are necessary. By increasing this number, only a very small influence on the hardness and heat resistance is exerted while on the other hand the tenacity is reduced.

As to the increase of tenacity, the elements tungsten and vanadium are found to provide favourable results, and possibly also a high chromium content.

It is a further object of the invention to provide a high tenacity chromium-tungsten-vanadiurn steel alloy, which is particularly suitable for cutting tools for machining otf chips, comprising 13-20% chromium, having the tungsten-vanadium content, taken as a whole, less than 50% of the chromium content, the tungsten content being 46% and the vanadium content being from 1.1 to 1.5%, all referring to the total composition of the alloy. Further the carbon content of the alloy should be above 1%, preferably between 1.1% and 1.4% according to the amount of chromium and vanadium present.

Therefore the alloy according to this invention does not contain other alloy components apart from chromium, tungsten, vanadium and carbon, and in order to have good fluidity, a small amount of a de-oxidizing element, such de-oxidizing element being preferably manganese or titanium.

The use of steel alloys for tools for machining off chips allows the designing of cutting angles which are substantially greater than those which were possible with the corresponding well known steel alloys. As is well known, the greater the cutting angles the greater the removal of the chips. Vice versa large cutting angles weaken the edge rendering it too acute. With tool steel alloys which do not possess any substantial tenacity, when cutting with the usual required speeds, breakages of saw tooth form soon occur on the cutting edge, while the cutter body itself breaks under the minimum increase of working speed or due to shock.

In Fig. 1 a milling cutter tooth is shown manufactured with the alloy according tothe invention, having a large cutting angle, while in Fig. 2 a milling cutter tooth is shown having a smaller cutting angle, as was usual with tool steel alloys used up to now. On comparing Figs. 1 and 2 it can be seen how the cutter tooth With the smaller cutting angle is weakened as compared with that having the larger cutting angle on the cutter body. For this reason such large cutting angles can only be made on tools made from steel alloys having a great tenacity such as those disclosed in accordance with the present invention.

Fig. 3 shows graphically how the temperature in C. of the cutting edge depends on the cutting angle in angular degrees, so as to illustrate why tools with a greater cutting angle give a higher output of cutting as compared with those having a smaller cutitng angle. It is seen from Fig. 3 that, for example, with milling cutters manufactured from the alloy according to this invention, which have a 20 cutting angle, the cutting speeds being equal, much lower temperatures occur as compared with milling cutters manufactured from the alloys used today, which have an 8 cutting angle.

With tools manufactured from alloys known heretofore, a temperature up to about 550 C. can be reached on the cutting edge damaging the edge. From the graph shown in Fig. 3, it results that, for example, having a cutting angle of 8 such a temperature of 550 C. which is still allowable is reached on the cutting edge, while with a tool manufactured from the steel alloy according to the invention having a 20 cutting angle, with the same working conditions a temperature on the cutting edge of hardly 220 C. occurs. Thus the machining speed can be stepped up with this last tool until it reaches the maximum allowable temperature of 550 C. on the cutting edge.

I claim:

A steel alloy suitable for tools having high tenacity in association with normal hardness, and normal resistance to wear and heat, said alloy containing as a basis a mixture of iron and chromium with the chromium content between 13 and 20%, additions of 4 to 6% of tungsten and 1.1 to 1.5% vanadium, with the combined contents of tungsten and vanadium being less than 50% of the chromium content, and having a carbon content between 1.1 and 1.4%, said alloy being based on a mixture of iron and chromium and containing besides the tungsten, vanadium and carbon no elements other than a small amount of a deoxidizing element such as manganese, titanium, and impurities.

References Cited in the file of this patent UNITED STATES PATENTS 

